The pronunciation of the possessive 's depends on the preceding sound:
1 - If the preceding sound is a voiceless consonant except /s/ /S/ /tS/, then 's is pronounced /s/
2 - If the preceding sounds is a vowel or a voiced consonant except /z/ /Z/ /dZ/, then 's is pronounced /z/
3 - After /s/ /S/ /tS/ /z/ /Z/ and /dZ/ it is pronounced /@z/
So: james's /'dZeImz@z/ actress's /'&ktr@s@z/, etc.