DS Rory Maxwell in Liar on ITV is played by Danny Webb.
He is on the case alongside DI Harmon (played by Shelley Conn).
Where have I seen him before? Danny Webb played Mortimer in The Halcyon, and Sydney Garin in SS-GB. He starred as Hobb in the TV series Humans. The actor has a long CV including a stint in Brookside in 1982 as well as Our Friends in the North, A Touch of Frost, The Young Indiana Jones Chronicles, Midsomer Murders, Silent Witness and Shackleton to name but a few.
Sci-fi fans may also recognise him from two episodes of Doctor Who; The Impossible Planet and the Satan Pit.