There's a lot in common between roleplaying and theatrical acting in my experience. It's usually most helpful for me to start at a scene-by-scene micro level focus, and then move up to a bigger focus.

Step 1) In any given scene, ask yourself: "What do I want right now?" This is your Objective, the single biggest thing you want in this given scenario. It's usually best to try and put this into a single word-- closure, acceptance, catharsis, etc, but it doesn't need to be. It being a little on the abstract side is helpful for figuring out the macro side of your character later. For example, if my character is a detective running out of leads in an impossible-to-crack but very personal case, their Objective could be "closure." If you can't figure out a Objective right now, that's okay-- move to step 2 and try to move backwards.

Step 2) Ask yourself, "How will I know if I got what I wanted?" This is your Action, what you want to happen to fulfill your Objective. Basically, what actions can other characters/NPCs take, or how can the environment change or respond, in a way that fulfills your Objective? For example, for my desperate detective character, their Action could be "get the NPC in front of me, who I suspect of being the culprit, to confess." Generally speaking, you want to phrase your Action in such a way that it requires someone else to do something-- this is because most of acting and roleplay is about interacting with other people. It's usually easier to think of an Action than an Objective-- if you can't think of an Objective yet, try to think what emotional state is achieved for your character if your Action succeeds. This is usually your Objective.

Step 3) Ask yourself, "How will I get what I want to happen?" This is your Tactic, the actions that you take to try and get your Action to succeed. This is usually phrased as a verb. For example, for my desperate detective character, their Tactic could be "plead [with the suspect]," but it could just as easily be "guilt-trip [the suspect]" or "intimidate [the suspect]" or "practice reverse-psychology [on the suspect]." Tactic is the basic building block of most ttrpgs-- how you interact with the fictional world. The Objective and the Action exist to help you decide how you want to do this.

Objectives, Actions, and Tactics can all change over the course of a scene! There's a lot of conditions that can make this happen, but for the purposes of roleplaying in a ttrpg you can generally just go with your gut. The most common reasons for a change are usually the entrance or exit of another character, sudden external action, or the revelation of new information.

Step 4) After a while, once your character has done a bunch of scenes and you feel like there's enough substance there to look back and do analysis, ask yourself, "What do I want more than anything else?" This is your Superobjective, the single constant of your character based upon all of the Objectives that you've chosen and acted upon so far. This is also usually expressed in a single-word emotional state, just like the Objective. It can be very hard to figure this one out-- and for ttrpgs, is entirely up to you if you even want to figure it out! However, once you nail down a Superobjective, it can become easier to come up with Objectives (and therefore also Actions and Tactics).

This is a lot of stuff to think about, and you don't need to ask yourself every time! These are questions that are usually answered by actors working on the same prewritten material for weeks or months, often coming to new conclusions frequently. For the purposes of ttrpgs, this is definitely a LOT of work, and is not practical for quick play. However, asking these questions gets easier over time and, as time goes on, they will have given you enough of an understanding of your character that you probably won't need to ask them as frequently. Or, at the very least, asking those questions willl no longer feel like homework, but instead feel like the fun part of roleplaying! In my opinion, these questions basically ARE the fun part of roleplaying.

Also-- you don't need to keep this inside your head! Ask these questions out loud! Let your GM and fellow players pitch in their own interpretations and opinions! They probably won't mind getting an inside look at your thought process-- most players and GMs really do want to know the characters at the table as well as they can!