"I think I've got something when I'm onscreen, but that's nothing to do with acting or talent"

Julie Christie is suggesting that she has a certain presence when she is onscreen, however that this presence is not necessarily related to her performing ability or skill. She suggests that her presence is something else, something that is not necessarily related to her performing abilities. This could be interpreted as her having a specific charm or appeal that is not always related to her performing ability. It could likewise be translated as her having a particular level of popularity or recognition that is not always associated to her acting ability. Whatever it is, it is something that she has when she is onscreen, and it is something that she values.