Graham Chapman did not receive an Oscar nomination for portraying Brian Cohen and Biggus Dickus in Life of Brian.
Life of Brian has plenty of absurd characters to go around played by the Monty Python members, but one of the least insane characters is played by Graham Chapman in the leading role. Brian is just a rather simple fellow and Chapman plays him as such going the extra mile really in his method of portraying that Brian is just some average sap. Chapman makes all of his expression simple without being overly stupid and his whole manner gives Brian a whole sort of naivety as a man. Brian is a man without really any big ideas or big ambition for himself therefore he's the perfect sort to be manipulated by some other upstarts. Chapman certainly gives us this man completely, and Chapman does a pretty admirable job of simply being the somewhat hapless fellow that Brian should be.
Most of the Python members are going for the laugh in a very broad way, Chapman does this himself which I will get to, but that's not the case with Chapman's portrayal of the lowly Brian. Instead Chapman is the straight man to all the absurdity as the film covers his reactions to those around him who seem rather insane. Chapman certainly derives a nice amount of humor as first Brian is just sort of strung long by the schemes as others as he basically smiles and follows like a good lad. As the film progresses Chapman adjusts accordingly to the more traditional straight man of comedy as he gets increasingly exasperated at the foolishness of others. Chapman again is quite funny and aids in the less subtle performances well with his own more down to earth portrayal of Brian's confusion and eventually annoyance at the actions of others.
Chapman is not only Brian being the straight man but he actually has his own broader character whose absurdity obviously begins with his name Biggus Dickus. Old Dickus is a Roman centurion who is not particularly intimidating to say the least. Chapman is fairly entertaining in giving the character a funny voice that sounds more like out of cartoon than a biblical epic. Overall though there really is not all that much of the oddly named man, and really Chapman's total time as this character is extremely brief with only a few lines given to him to say in that funny voice. Also I won't lie though these scenes though do belong to Michael Palin as Pontius Pilate not Chapman. Chapman's never bad though and certainly gets his fair share of laughs too, but Palin's performance is the one that you remember.
Chapman overall succeeds quite well within the film being the appropriate element of sanity to stabilize the humor in the right fashion when playing Brian, and being properly entertaining with the one character that let's him join in with the rest of the cast. I would not say that this is a great performance by any measure as I don't think this is a case of straight man turn that actually steals the show from the showier turns, in this case it is the showier turns that are the funniest, and even Chapman's showier character is far from the funniest character of that nature. This is a good comedic performance that serves his purpose well enough but I never thought it was a truly hilarious performance nor did I feel that Chapman went all that further with any other element of the character to make up for the fact that he's not ridiculously funny.