There are a number of conflicting goals when trying to design and roleplay a character's motivations and personality.
Gempunks is frequently a game about solving problems with fantastical tools. If you play character(s) who act less intelligent than yourself, your ability to find the most impressive or interesting solutions may be impaired.
Be Yourself, Somewhat
If you find yourself discarding your best ideas because your character would never think of them, your character won't seem very clever. If you try to play a character with somewhat different priorities and justifications from your own, however, you may find that you can think in more ways than you thought you could, so don't be too timid about this.
Attempting to carefully avoid any non-character knowledge when making a character decision may sound noble, but it's entirely sensible for your character to be making decisions using knowledge you don't have, and this careful avoidance is likely to end up preventing you from making decisions that you would have made even without the out-of-character knowledge. As a compromise, you could make the decision you were going to make, and then figure out what your character must have thought instead in order to make that same decision.
For characters that are supposed to be smarter than yourself, it may even be reasonable to take suggestions by other players and use them as the character's own thoughts.
Roleplaying is an opportunity to think and act like someone else.
Have a Backstory
The act of writing a backstory for the character fleshes them out in your mind, making it easier to think like they do in a way that's internally consistent. It also allows you to make references to things the character has experienced in the past in a way that implies that they existed before the campaign started.
If the backstory makes assumptions or declarations about the setting, share it with the DM so they can incorporate it, and so they can tell you which details contradict setting details they care about.
Figuring out how a character would act if they didn't know what you know can be a challenge, but doing so makes the character that much more distinct, and occasionally injects a healthy dose of dramatic irony into the game.
Being Amusing, but not Annoying
An RPG can be an excellent venue for situational improv comedy, but that's usually not the only thing all the players want from the game.
Have a Good Sense of Humor
Following any steps presented here will not magically make you funny.
When the game is at a point where something serious is happening, or you're simply not thinking of any good jokes right now, you should be able to do something sensible instead. Wasting the actions of a major part of the team for a practical joke in the middle of combat is very likely to bother at least one other player.
Some of the most memorable and impressive deceits happen when players trick each other. Ethical discussions from the viewpoints of culturally distinct characters can be very interesting too.
Being a Traitor
There are many kinds of traitors: The evil mastermind kind, the mercenary kind, and the crazy kind, for example.
The evil mastermind kind is more impressive if you can reveal that you've been working against the party for a while. The other party members usually have every right to kill you if they catch you in the act; when it eventually becomes clear that you're an enemy you should have escape routes or secret countermeasures ready for them. After the conflict you should switch to actually trustworthy character(s), because the other players will presumably try not to be so gullible again.
The mercenary kind is where you betray the rest of the party when you get a sufficiently intriguing offer. Perhaps the other people in the group will try to buy you back, or maybe they will just destroy you. Maybe this gives the party an opportunity to win your character back and convince them to be a slightly better person (character development!), maybe this is a way to help the DM introduce a new sidekick villain, but probably the other people will decide they don't like working with mercenaries.
The crazy kind is where you betray the rest of the party, after making it totally clear that you're the sort of person who might do that. This calls into question why they let you join the party in the first place. If the objective was to make people think more carefully about making characters who have a reason to work together, this may succeed, but it's not likely to make people very happy.
Have a Reason to Cooperate
There is a wide range of setting-appropriate cultural upbringings to choose from in a setting, and some of them hate each other. Having dissenting viewpoints is good, but in the end, you're supposed to mostly work together, so it doesn't make sense to have the complete spectrum of perspectives in a single group.
Consider an extreme example: you want to play someone who hates mages, but there are already spellcasters in the party. Perhaps your character is open-minded, and willing to be persuaded, during the session in which they're introduced, that not all spellcasters are always evil. Maybe they like to proselytize, and are willing to work with these people to persuade them that they should stop being mages. Possibly they simply have an goal so difficult that they need magical assistance.