When a notification with actions is displayed, the actions are available as buttons (for example, "Previous" and "Next" in a VLC playing notification) by mousing-over the notification. However, after a few seconds the notification disappears, and is only displayed in the overview (from clicking the clock in the top panel). This shows the notification that was received, but it only has a close button to remove it. The text of the notification is truncated, and the action buttons are removed, and as far as I can tell there is no way to get either of them back; therefore if you wanted to click one of the actions or read the full text you have to do it in the few seconds before a notification is collapsed into the overview.