When PlayStation 4 launched, consumers were led to believe that the console's 'Interactive' streaming wasn't active yet. By that logic, viewers wouldn't be able to interact with your game, taking over when you let them or influencing what happens through comments.
Either 'Interactive' streaming has actually been enabled from the start, or it was quietly added in a firmware update. It's working--it's just up to developers to actually use it.
According to Sony Worldwide Studios president Shuhei Yoshida, Tecmo Koei's Dynasty Warriors 8: Xtreme Legends uses it in Japan. Players can type particular messages in the comments section to, for example, drop healing items for the player or add more enemies to the fray.
In North America, a few NeoGAFers have already noticed the feature's presence in early streams of Tomb Raider: Definitive Edition, which went on sale at GameStop ahead of its planned January 28 release. Myriad commenters have taken to typing 'DIE' and 'REVIVE' while folks are playing the game, but it's unclear what effect, if any, this has on gameplay. One poster quoted the following as a suggestion: "Continually chanting DIE in Twitch chat causes Black Phantoms to appear in Laura's world, adding difficulty". Dubious mispelling of Lara notwithstanding, no source is cited.
Yakuza Ishin (Japan-only) will also makes use of Interactive features. According to Gematsu, viewer comments appear in-game and can influence how quickly you recover from being knocked down and how fast you build heat.
Sony has yet to offer comment beyond Shuhei Yoshida's tweet, but you'll know as soon as we do what 'Interactive' streaming truly entails and in what capacity it's already available.