The two principals are perfectly cast. Never let it be said again that Hugh Grant always gives the same performance! In "Scandal," he's still in fine post-comeback form, the annoying mannerisms of his youth--head ducking, stammering, forelock tossing, sheepish grin--all forgotten. It may have been a stretch for him to play a dark and stormy guy like Jeremy Thorpe, an ambitious pol who took out a hit on an inconvenient ex-lover, but he turns in a fascinating, expressive performance. In a scene where a political rival has just attempted to do him in, we loved the way Grant switches from affable condescension (default setting) to venomous contempt in the blink of an eyelash.
Ben Wishaw's clearly the go-to guy for a character who's gay, fey or just very sensitive (Richard II, Paddington, Keats). He shows us Norman Scott, the target of Thorpe's shambolic murder plot, as, sequentially, a needy, hapless drifter, kept man in a Chelsea bedsit, Carnaby Street strutter, hapless drifter again (but catnip to a lusty Welsh widow and a motherly pub owner), and finally, if a bit contrafactually, a hero of gay liberation. (In the film, he gives a stirring speech on behalf of "men like me" from the witness box at Thorpe's trial, but IRL he was so shy and nervous that his voice was barely audible in the courtroom.)
In fact, Wishaw's performance may come off at times like an anachronistic portrayal of a sexy, gender-fluid 21st-century dude, and the real Norman Scott apparently objected to being made out to be "this poor, mincing little gay person," but Wishaw's character's odd mingling of the pitiful and seductive makes him the perfect foil for Thorpe's Old Etonian sociopath.
The script practically writes itself when it comes to the murder plot and the courtroom scenes (too bad Peter Sellers was born 50-odd years too soon to play the hitman manqué). The supporting cast is very strong, with great actors in even the smaller roles--fabulous Patricia Hodge gets the last word as Thorpe's mother. Don't miss the clip during the credits from Peter Cook's parody of the judge's outrageous charge to the jury ("a self-confessed player of the pink oboe"); the whole routine, largely improvised onstage for The Secret Policeman's Ball, is worth seeking out on youtube.