How are abilities unlocked in the Witcher 2?

On the character attributes page is a section for abilities (different from talents). I've earned the abilites Axii sign hex, cover, conjuror and assassin. Each of those gives a certain bonus, which is nice, but I have no idea how I unlocked most of those abilities.

I got the Axii sign hex when I first used Axii in a conversation, but I've no clue how I unlocked the rest of them. Are they given to you on certain events, or do you have to do something special to get them?

These abilities are gained by performing specific actions, but they're not revealed to you until after you've done them. Some examples:

• As you mentioned, Axii is received after first using it in conversation.
• Assassin is received after locating Altair's body in the prologue.
• Cover is received by staying under Triss's magic shield for the duration of that event at the start of chapter 1 (source).

Some of these are possible to miss, so it pays to be an explorer and a completionist.