ONE OF cinema’s most mercurial and influential talents, Robert Redford is an awfully difficult fellow to pin down.

A 1970s pin-up turned Utah recluse, a horse opera gunslinger turned eco-activist, an actor turned director, Redford has always had a knack for defying expectation. He's the star who followed The Great Waldo Pepperwith All the President's Men, the film-maker who knocked together the sublime, taut Quiz Showbetween the collective sprawl of A River Runs Through Itand The Horse Whisperer.

The Conspirator, a courtroom drama, marks a considerable return to form after 2007's lacklustre, Bush-bashing Lions for Lambs. Placed at a safe historical distance from any contemporary imperial unpleasantness, the new Redford flick finds a less didactic means of tub-thumping by exploring the parallels between recent suspensions of human rights and the aftermath of Abraham Lincoln's assassination in 1865.

Redford’s undiscovered historical country is the trial of Mary Surratt (Robin Wright, excellent, chilly). As the owner of the boarding house where John Wilkes Booth colluded with his Confederate co-conspirators (among them her own son), Surratt was imprisoned and tried by a military tribunal along with seven other suspects.

Enter Frederick Aiken (James McAvoy), a dashing Yankee captain and aspiring attorney cajoled into handling Surratt’s case by his mentor (Tom Wilkinson). Aiken is plainly appalled by what this southern Catholic woman may or may not have known about her offspring’s activities, but he’s even more aghast at the travesty that awaits her in the makeshift courtroom.

McAvoy, a likeable, passionate hero, is in good company here. Indeed, what The Conspiratorlacks in spectacle it atones for with starry, thespian clout; Kevin Kline is a hawkish Edwin M Stanton, Evan Rachel Wood is stoical as Surratt's persecuted daughter, and Colm Meaney presides dutifully and grouchily over the trial.

The stagebound nature of the material suits Redford's directorial leanings. Always at his best when working within small narrative and spatial confines, the 75-year-old film-maker regains some of the focus found under the lights of Quiz Showor stuck between Mary Tyler Moore and Donald Sutherland in Ordinary People. It's almost another comeback for the Sundance Kid.