VIDEO game giant Sega Enterprises and movie distributor Shochiku Co plan to work together on developing new forms of entertainment. Sega said one project was a detective film that gave viewers a role. ''Rampo'', in which viewers helped characters make decisions mid-plot, would be released next June, a spokesman said. The companies said top executives were discussing how to arrange the alliance. A joint venture company might be formed to develop and give a brand name to new television and publishing characters and products. They refused to say when the tie-up would be officially announced, or what other products were being considered. Sega is Japan's leading arcade video game-maker and is second to Nintendo in the world home video game market. Exports account for more than 60 per cent of its revenue.