This critically acclaimed awward-winning drama series has sometimes been criticized for being too formulaic. While its writers and producers have defended this by arguing that the formula is less important than the specific elements of each case presented in the various episodes, it is hard to deny that there is certainly a formula in place and that anyone familiar with more than a few episodes is probably very much aware of it.

The anatomy of a typical CSI episode

Most episodes start just prior to the crime that will form the basis of that episode. We are often introduced to someone who is about to become a victim or, sometimes, when the writers want to switch things up a little, otherwise uninvolved citizens discover a body or crime scene. When the viewer does witness the crime, the details provided are sparse. The show does not make extensive use of dramatic irony, and viewers typically learn of new developments as the investigators unearth them.

When the episode opens with a soon-to-be-victim in the moments leading up to the crime, it is also common to not see the crime actually occur. We often see the seconds just before the crime, then a quick cut to after the crime has taken place. After the "jump," the CSIs are usually just arriving at the scene or have been there for a very brief period of time. They frequently interact with David, the coroner's assistant, who provides information regarding time and cause of death. The CSIs/detectives on the scene then make some observations regarding the crime scene, and one of them (usually superviser Gil Grissom) makes some kind of remark that sums up the scene, usually involving some kind of pun.

The opening credits are then played. Unlike the majority of other television shows, they always come after the scene that sets the tone for the rest of the episode, even when the rest of the episode deviates from the standard CSI formula. The credit sequence features clips from previous episodes accompanied by the Who's "Who Are You?" ("Baba O'Reilly" in CSI: New York and "Won't Get Fooled Again" in CSI: Miami).

The main actors are given prominent billing in the main credit sequence. Supporting actors, including those with recurring roles, are billed during the secondary credits shown over top of the episode's next scenes. Recurring actors are billed first, then all other supporting actors are billed afterwards. The lone exception to this is when very famous people (think Faye Dunaway) guest star on the show; they are often billed before the recurring actors. (It should be noted, however, that when Roger Daltrey guest starred on the show he was credited after later-to-be-main-actor Wallace Langham as a "special guest star.")

The scenes following the main credits are also often formulaic. There is often further analysis of the crime scene, featuring the introduction of preliminary suspects and the procuring of trace, DNA and fingerprint-related evidence, which is sent to the lab's various technicians to be analyzed.

It is common practice for the investigators to begin suspecting the most "obvious" suspects early on in the episode; this is often (but not always) an indication that the person is actually innocent. These suspects are interrogated and maintain their innocence and are rarely believed by their investigators until one of them, or one of the lab technicians seen throughout the show, provides test results or evidence that exhonerate that suspect, sending the investigators back to square one.

It is rare for every CSI to be involved with one case. More often than not, three investigators work on the "main" case (known to some fans as the "A plot") while the other two work on a secondary case that usually has nothing to do with the main case (the "B plot"). The two plots are presented simultaneously (that is to say that one case isn't solved long before the other), with scenes from each presented interchangably.

Most episodes often see the CSIs having to return to the crime scene to re-evaluate the evidence they've collected, or to search for more. Among the show's regular themes is that of changing one's initial theory as new evidence comes to light. Sometimes the two plots are connected, but more often than not they have nothing to do with each other.

As the evidence comes together, there is usually one bit of information -- most often DNA-related -- that breaks the case open. It is, naturally, discovered towards the end of the show and usually leads to the questioning of someone you never really thought to suspect. In many episodes, this person confesses during interrogation; in others, he or she maintains his or her innocence but it is clear that he or she is guilty.

The typical final scene of a CSI episode is often (though not always) more light-hearted, sometimes allowing us a glimpse into the personalities of the characters. It also often revolves around the circumstances involving that particular case, with the characters sometimes reflecting on what they would have done had they been in situations similar to the victims or even the assailants.

Exceptions

Special episodes

Some episodes get as far away from this formula as you can get. These, often involving big cases, rarely involve a secondary plot and often see the entire team investigating one case or may even revolve around members of the team testifying at the trials of those charged with crimes based on the evidence they gathered.

Though rare, there are episodes intended to further character development as well. The fifth season episode Nesting Dolls included several seasons providing information about Sara's family history. While character development is most often included in the more formulaic episodes in bits and pieces, some episodes break away from the formula in order to teach viewers more about the characters. Other examples of this include The Unusual Suspect, where the entire team (or, at least those who appear in the episode) is focused on one case and we learn more about the characters than we might in most other episodes.

Cliffhangers

Several episodes are "two-parters," where the case goes unsolved in the first half and is eventually solved in the second half. Examples of this include the seventh season finale Living Doll and the eighth season premier Dead Doll, where Sara is trapped under a car in the desert by the Miniature Killer; Grave Danger parts one and two (directed by Quentin Tarentino) sees Nick kidnapped and buried alive while the other CSIs work tirelessly to save him (and not on any other cases) is also a two-part episode. Unlike Living Doll and Dead Doll, both parts aired at the same time. At the end of season six, Brass is shot while trying to diffuse a hostage situation.

The Miniature Killer

The seventh season featured a number of episodes dealing with a psychotic serial killer who placed extremely detailed scale models of the crime scenes at the crime scenes. This storyline involved several victims over several episodes and therefore didn't fit the generic formula, though the first few episodes that dealt with these cases were in that format (with a secondary case, etc.). This was the show's longest storyline to date, spanning from the first episode of season seven to the first episode of season eight. 

Etc.

Is any of this surprising? Not really. Murder mysteries have always been reasonably formulaic, and CSI is really just a murder mystery series with lots of science. The idea that the murderer is always someone you least suspect is as old as time itself, and was just as prevalent in shows such as Murder She Wrote and so forth.

Furthermore, formulaic plotlines have also been an element of TV shows for years. Full House episodes are just as formulaic. The challenge for the writers is to make the content of every episode as different and interesting as possible, which may in fact only be possible because they often use a set formula.


References:
http://www.csi.com
http://en.wikipedia.org/wiki/CSI_episodes
CSI seasons 1-7 (DVD).