During the mission where you're auditioning for a role in a film (I won't say where), you'll meet the fellow you need to kill. During the audition, you'll be asked to stand up and physically act out part of a scene. Your target will be lying there on the ground right in your path. Simply walk up to him, hit
, and the achievement is yours (you'll die and go back to a checkpoint, but I think that's worth 10 GS).