According to their source, Carnage will, unfortunately, have no part to play in the main body of the film.
“We know that filming recently took place in San Francisco and it appears that near the end of the film, possibly in a post-credit sequence, Brock will find himself in San Quentin prison, located just to the north of San Francisco. At San Quentin, it appears that Kasady finds himself under the care of Dr. Ashley Kafka, who introduces him to Brock but cautions him to keep his distance. From what our source is telling us, it is during this meeting between Brock and Kasady that a portion of Brock’s symbiote begins making its way towards Kasady before we are cut to black!”
As for Harrelson, here’s some speculation (no, this isn’t a sequel to Life):
As for which actor could be playing Kasady, we don’t have any confirmation, but we can tell you this: back in December, Woody Harrelson was cast in Venom in a role described as a “henchman of sorts.” Despite numerous set pictures revealing most of the cast, not one picture of Harrelson on set ever emerged…
Credit Bloody Disgusting