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.
Often, the interactions can be quite diverse. For instance, a Space Marine might initially be very suspicious of the ponies due to their cute and seemingly harmless appearance. But as the story progresses, he might come to respect their strong sense of friendship and unity. Ponies, on the other hand, could be both fascinated and frightened by the imposing and often brutal Warhammer 40k characters. They might try to use their friendship magic to soothe or befriend them.
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.
They can interact in many ways. Sometimes they are enemies, fighting over resources or territory. For example, in some stories, humans might try to colonize a planet that the Eldar consider sacred.
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.
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.
The military - minded characters from Stargate, like Jack O'Neill, would likely be initially cautious and somewhat skeptical of the more diplomatic Starfleet officers. However, they would soon realize the value of Starfleet's knowledge and resources. On the other side, Starfleet officers might be intrigued by the Stargate technology which seems so different from their own. For example, Captain Kirk might be eager to explore through the Stargate.
In a crossover, the Elites from Halo might interact with the Vulcans from Star Trek. The Elites' honor - based culture could be compared to the Vulcans' logic - based society. They could start off as adversaries, perhaps misunderstanding each other's motives, but then gradually come to respect one another as they learn more about their respective cultures and values.
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.
The characters might initially be cautious of each other. For example, Shepard from Mass Effect could be skeptical of the Star Trek crew's non - violent approach at first. But as they get to know each other, they could start to share their skills. The Star Trek crew could teach Shepard about their advanced medical techniques, while Shepard could show them some combat maneuvers.
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.