Crook, who recently played a gangster in Skins' third series, guest stars in the opening episode as a creepy character called Cedric. Lester, meanwhile, will appear later in the series as a devious man named Myror.
Also appearing alongside the original cast will be Mistresses actress Sarah Parish, who plays a troll masquerading as a beautiful woman, and Gosford Park's Charles Dance, who joins the cast as a sinister witch-finder.
Merlin attracted more than 6.3 million viewers throughout its first series on BBC One and has since been picked up by international broadcasters.
The show's new 13-part series, in which the young sorcerer continues to conceal his powers, has been described by the BBC as "bigger and bolder" than before. It will include new villains, new fight sequences as well as continued work with the Oscar-winning visual effect team behind Doctor Who.
"With its combination of fantasy, humour and adventure Merlin has firmly established itself as a Saturday evening family fixture," the BBC's executive producer Bethan Jones promises.