It's not a binary spectrum between action game and walking simulator.
Tomb Raider had some sections that probably fit the walking simulator genre, but also areas of action and puzzle games.
Walking simulator is defined by moving and exploration. Interactions usually involve reading, or changing your environment (not in ways to unlock new areas, but perhaps to see new things). Playing one, you clearly note the vibe. There isn't much to do except move forward and see/experience new things, building out a plot and understanding of a narrative as you do so. You don't generally drive the plot through your actions, but instead learn about it.
Tomb Raider had some sections that probably fit the walking simulator genre, but also areas of action and puzzle games.
Walking simulator is defined by moving and exploration. Interactions usually involve reading, or changing your environment (not in ways to unlock new areas, but perhaps to see new things). Playing one, you clearly note the vibe. There isn't much to do except move forward and see/experience new things, building out a plot and understanding of a narrative as you do so. You don't generally drive the plot through your actions, but instead learn about it.