Forget most of what you've read about this movie. It is not postmodern, nor "Pythonesqe", nor any of the other adjectives I've read in user reviews. What it is is a very intelligent, and very different piece of filmmaking that is quite unlike anything you've ever seen. If I had to compare it to any movie in recent history, I'd say it's a bit like "Adaptation", but that film was crude and heavy handed in comparison to "Tristram Shandy". It's also a bit like "This Is Spinal Tap" in the deadpan way it presents some very silly parody.
Imagine a Merchant-Ivory costume drama in which the principal actor suddenly stops, turns to the camera, and tells a Groucho Marx story, And imagine that part way through an increasingly confusing narrative, that keeps movinng forward and back, in fits and starts, the camera abruptly pulls back, and we see a film being made.
From that point on, "Tristram" becomes the story of the attempt to make a film from a very difficult to film novel, with a very difficult cast. The lead hasn't read the book, and is consumed with petty jealosies concerning the main supporting actor. The producers don't want to put any more money into it. And it just gets sillier, and sillier- while never quite falling into slapstick.
In point of fact, as conditions around the film get sillier, the lives of the main characters become more complicated, and consumed by some very serious issues. And yet everything slowly comes togethers, a few people learn a few lessons, and a film is made... although no one seems to ber very pleased with the results. And then, as the credits are rolling, the two main actors are debating acting techniques, in what may be the funniest scene of the entire film.
So what we have here is a very complex film that requires close attention, and perhaps even a little knowledge of literature and history to get all the jokes. This is not simple minded slapstick, and those who prefer that sort of humor simply aren't going to get it. But for veiwers who bring with them the intelligence to apppreciate all the jokes and references, it delivers a very satisfying and laughter filled two hours.