If you've been paying attention to the marketing campaign for Avengers: Infinity War, you will have noticed that some characters have been notable for their absence. So far, we've not see the Black Order (other than in small glimpses) and there's no sign of Peter Dinklage's rumoured character or the Collector - all of whom are important to the plot.
More notable on the hero front is the absence of both Ant-Man and Hawkeye, who are both completely absent from the trailers released so far AND the new raft of character posters that has just been released.
Here's the posters...
Now, there's no Black Panther poster there either, but his role in the film is already assured, so it's likely he's going to get a character poster. It makes sense, in fact, that Marvel would release that - and some other Wakanda-specific marketing content - as a means to capitalise on Black Panther's success at the box office. So expect that soon.
Ant-Man and Hawkeye - on the other hand - are probably absent here because they're just not involved very much. If at all. And unfortunately, it wouldn't be illogical for that to be the case.
A leaked plot description from 2017, which hit 4Chan (so was somewhat inevitably cast off as fake) has called pretty much everything correctly so far, and that claimed Ant-Man wasn't involved in any major sort of way. And on top of that, Hawkeye only "turns up for the final battle" in Wakanda.
Unfortunately for fans of the characters, it makes a lot of sense: as the prelude comic for Infinity War confirmed, both of them return to their families in the wake of Civil War rather than staying on active duty with Captain America's underground team. And while Scarlet Witch does the same, she has a reason to be heavily involved, as she'll be with Vision in the first act.
Ant-Man and Hawkeye, meanwhile, have very little reason to be called back, even with a world-threatening force coming to Earth. Think about it: Thor and the Guardians will be in space, where they'll come into direct conflict with Thanos, Doctor Strange will be the focal point of the New York attack, which Tony Stark and Peter Parker will witness thanks to proximity (and Bruce Banner will join via space after encountering Thanos too). Cap, Falcon and Scarlet Witch will be forced to save Vision and Scarlet Witch and will head to Wakanda, dragging Black Panther into play. Bucky comes with Wakanda and War Machine will go where Iron Man goes.
Hawkeye will be on his farm, and Ant-Man will be on the other side of the country looking after his family. There is no call, at this point, for them to be involved. It's not like the Avengers are mobilised - they're attacked and each respond organically. They wouldn't call the two hire-for-help heroes who are both essentially retired for the good of their families.
It won't be until Avengers 4 requires Ant-Man's technology (or simply needs him and Wasp to make up numbers when other heroes fall) and something terrible happens to turn Hawkeye into Ronin that they become necessary characters. That's why they're not in the marketing - there's just nothing to market.