I would like some help with the actions being shown on the 3D viewport window. I imported a character from a different software, and I also imported several animations (.fbx) that I saved using different actions for each animation. However, even If I change the selected action in the Action Editor, the 3D viewport window will not show the action I am currently selecting. Is there a way to change the action shown on the viewport ?
Actions have to be applied on the proper object: the armature.
Probably you're applying the action to the mesh, while you should select the armature and then choose an action from the action editor dropdown menu.
In this way the action in the 3D view will reflect the choosen action, unless there is some NLA strip that overrides it. So check your NLA editor also, and eventually delete/disable all action strips in NLA.