Well, the races are very different. In Star Trek, you have the Federation races like Humans, Vulcans, and Klingons. They generally have a more diplomatic approach. In Warhammer 40k, races like the Orks are just out for a good fight all the time. So in a crossover, the Orks might see the Klingons as kindred spirits in a way, but the Klingons would probably be put off by the Orks' lack of honor.
Ancient humans in Warhammer 40k fanfic can be very different from modern races. In terms of culture, modern races are often more fragmented and war - like. Ancient humans may have had a more harmonious society. They also might have had different physical capabilities. Maybe they were more resistant to the harshness of space without the need for bulky power armor like the Space Marines.
Sometimes, the interactions are based on a need for mutual assistance. A Star Trek ship might find itself in the Warhammer 40k universe and need the help of a local faction to repair their ship. In return, they offer their knowledge or technology. This can lead to some interesting exchanges as the characters from both universes try to understand each other's values and capabilities.
One interesting concept could be the clash of different ideologies. In Warhammer 40k, there is a lot of grimdark and a strict hierarchy, while Star Trek is more about exploration and unity. So seeing how characters from both universes react to the other's values can be really engaging.
The gods in Warhammer 40k are on a whole different level of power. They can warp reality on a galactic scale. In Marvel, the Asgardian gods like Thor have great power, but it's not on the same scale as the Chaos Gods in Warhammer 40k. But if Thor were to face off against a daemon from Warhammer 40k, his lightning and strength might be enough to hold his own, especially if he had the help of some of his Marvel comrades and the technology and weaponry from Warhammer 40k.
In combat situations, they could either be aggressive or form alliances. If they see a race as a threat, they will fight them. But if there's a common enemy, like a chaos invasion, they might team up. For example, they could work with the Eldar, sharing battle strategies and combining their unique abilities to defeat the greater evil.
A common crossover is the appearance of Starfleet officers in the Warhammer 40k universe. They might be seen as strange and otherworldly beings. Their peaceful exploration ethos would be a sharp contrast to the constant warfare in 40k. For example, they could try to broker peace between warring factions in 40k, which would be a huge challenge given the nature of that universe.
In 'Ex Machina', the characters are mostly dealing with the implications of creating and interacting with an AI. Their personalities are often defined by their relationship with this new form of intelligence. In 'Star Trek' novels, characters come from diverse species and backgrounds. They have different values and beliefs based on their home planets or cultures. 'Ex Machina' characters tend to be more on the edge of discovery in a very specific technological area, while 'Star Trek' characters are exploring a vast universe of different ideas, technologies and cultures. For instance, a 'Star Trek' character might be a Vulcan with their logical mindset, which is very different from the mostly human characters in 'Ex Machina' who are more driven by human curiosity and fear in relation to the AI.
They could be very cautious. Since Xenos races are often unpredictable in the Warhammer 40k universe, the new human faction might start by observing them from a distance. Maybe they send out small scout teams to learn about their behaviors and capabilities.
In Warhammer 40k, the Emperor is an extremely powerful character. His psychic powers are off the charts and he has a huge influence over the entire Imperium. In a face - off against Star Trek characters in fanfiction, he would be a formidable force. In Star Trek, Q is a very powerful being. He can manipulate reality at will. If Q were to engage with the Emperor, it would be a battle of two extremely powerful entities with different natures. Q's mischievous and almost omnipotent nature against the Emperor's more serious and controlling power would be quite a showdown.
The space battles are often the highlight. Warhammer 40k has huge fleets of gothic - looking ships armed with massive guns and torpedoes. Star Trek has its own sleek starships with advanced weaponry like photon torpedoes and phasers. In fanfiction, a large - scale battle between a Warhammer 40k imperial fleet and a Star Trek Federation fleet would be amazing. The different tactics, ship designs, and crew capabilities would all come into play. For instance, the hierarchical and often fanatical nature of the Warhammer 40k crews compared to the more diverse and diplomatic Star Trek crews would lead to very different decision - making processes during the battle.