If you're stuck, go back to important scenes and drag your finger methodically across the screen to ensure that you haven't missed any objects. Additionally, go through every available option - examine, interact, etc. - to make sure you haven't overlooked anything.
Typing in keywords can be done by selecting the typewriter icon when engaged in a conversation. Fortunately, you won't have to use it to drive conversations forward.
Words coloured green can be tapped to trigger additional conversation topics. Make sure you're selecting them to move the game forward.
To pair items in your inventory with those in a scene, drag the object to the lower-right corner to close the inventory and then onto the hot spot.
Objects in your inventory can be examined for bits of vital information. Additionally, you can access Louis's mobile phone to place calls and answer messages.