[5] After training at Central School of Speech and Drama, Gwillim began his acting career in earnest in the 1950s, working on both stage and screen.

[6] He performed in an extensive amount of theatre, both classics and modern plays, in the West End of London and on Broadway.

[5][7] Some of his most notable roles include: playing in Ralph Richardson's production of The Merchant of Venice; The Right Honourable Gentleman with lifelong friend Anthony Quayle; a revival of My Fair Lady with Rex Harrison, playing Colonel Pickering; John Gielgud's The Constant Wife, with Ingrid Bergman; and The Iceman Cometh, with James Earl Jones.

[4] His military background, commanding presence and deep, booming voice typecast him as soldiers and authority figures.

(1960), the archbishop Hubert Walter in Sword of Sherwood Forest (1960), the obnoxious club secretary in Lawrence of Arabia (1962), King Aeëtes in Jason and the Argonauts (1963), an RAF officer in the James Bond film Thunderball (1965), the Lord Chief Justice in A Man for All Seasons (1966), General Harold Alexander in Patton (1970), Poseidon in Clash of the Titans (1981), and Van Helsing in The Monster Squad (1987).