▁the ▁complexity ▁of ▁Gr iss om ' s ▁character ▁and ▁he ▁decided ▁to ▁aud ition . ▁It ▁was ▁the ▁second ▁time ▁Peters en ▁port rayed ▁a ▁for ens ic ▁scient ist , ▁having ▁port rayed ▁Will ▁Graham ▁in ▁the ▁ 1 9 8 6 ▁film ▁Manh unter . ▁ ▁Development ▁ ▁Gil ▁Gr iss om ▁first ▁appears ▁in ▁C SI : ▁Crime ▁Sc ene ▁Investig ation ▁in ▁the ▁pilot ▁episode . ▁After ▁this ▁he ▁appeared ▁in ▁almost ▁every ▁single ▁episode ▁of ▁the ▁show ' s ▁first ▁eight ▁years , ▁except ▁in ▁" Hol lywood ▁Br ass ," ▁from ▁season ▁five , ▁an ▁episode ▁that ▁turns ▁entirely ▁around ▁Jim ▁Br ass ; ▁besides ▁him , ▁only ▁three ▁other ▁regular ▁characters ▁appear . ▁William ▁Peters en ▁did ▁not ▁appear ▁during ▁the ▁season ▁six ▁episode ▁" G um ▁Dro ps ." ▁This ▁episode ▁was ▁originally ▁going ▁to ▁be ▁how ▁Gr iss om ▁was ▁convinced ▁that ▁an ▁ab du ction ▁victim ▁was ▁still ▁alive . ▁When ▁Peters en ' s ▁nep hew ▁died , ▁he ▁fle w ▁home ▁and ▁was ▁un available ▁for ▁the ▁film ing ▁of ▁the ▁episode , ▁which ▁was ▁re written ▁to ▁be ▁centered ▁on ▁Nick . ▁During ▁ 2 0 0 7 ▁( C SI ▁season ▁seven ), ▁William ▁Peters en ▁took ▁a ▁break ▁from ▁C SI ▁to ▁appear ▁in ▁a ▁five - week ▁run ▁of ▁the ▁Tr inity ▁Re pert ory ▁Company ▁production ▁of ▁Dublin ▁Carol ▁in ▁Prov idence , ▁Rh ode ▁Island , ▁resulting ▁in ▁Gr iss om ▁taking ▁a ▁s