I'm not generally a fan of the spy genre. When I was in middle school I started reading some of the James Bond novels - not the originals, the later ones that were current at the time. They were quite similar to the Star Trek adventures, which is to say palatable only to someone who didn't know better.
I bring this up because I've never read a John le Carre novel and don't think I ever will. I have a sense it would likely be a let-down because the Alec Guinness adaptations are simply so good.
Indeed, Guinness inhabited the role of the protagonist, George Smiley, to such an extent that the author himself modified his character based on the portrayal.
Basically, Guinness had the definitive take, not le Carre.
It happens from time to time. I'm told that Tom Selleck basically defines the Jesse Stone literary character because of his superb portrayal.
One could of course mention Vivien Leigh and Clark Gable defining Scarlett O'Hara and Rhett Butler, but even in our modern age a well-done adaptation can shape the source material.
So it is that I'm watching Tinker, Tailor, Soldier, Spy and will then move onto Smiley's People, both of which I own on DVD.
The plots in both are intricate and interesting, but the actor's performances are what really get me to watch it again and again. They are superb.
I think that's really the biggest factor in the success of a film adaptation of a book. The plot being mulched is also a risk, but it can survive if the actors are compelling enough.
Remember, the Golden Age of Hollywood wasn't about superior stories, it was about larger-than-life performances. Sometimes, as in the case of Clark Gable or John Wayne, you weren't looking to see a story so much as a known actor do his thing. Gable didn't have immense range, but the man had vast reserves of presence and charisma.
Guinness is the opposite - seemingly unassuming, he can shift form like a chameleon, equally at home as a brash British officer, a Soviet KGB general or an Arab sheik. In George Smiley, he gets one of his best roles, because Smiley himself is a master of deceit, and can be both quiet and meek as well as cunning and vicious.
It's a pleasure to watch and the written version just can't compare.