Use the mouse to control Rufus. Use the scroll wheel to open the inventory. Press LMB to pick up items. Press RMB to examine them. LMB is also for interacting with NPCs. Press the scroll wheel or the space key to highlight hotspots. The Esc key is for opening the game's menu, skipping cut-scenes and dialogs. If you are controlling more than one character, switch between them by clicking on the icons at the bottom of the screen.
The game has various mini-games, but you can skip them - do that by pressing the red cross in the right corner of the screen.
Items that should be added to Rufus' inventory are marked red. The correct dialog lines are bolded.