A thief's surprise and backstab ability revisted

[AxeMental]

I was wondering how many would agree with this interpretation of the PH description of back stab: 1. The thief must attack from behind with a handheld weapon unnoticed, 2. a MS role allows the thief to sneak up behind their prey, or a HIS allows the target to pass by the thief who can then back stab. 3. A MS or HIS role made does not guarantee surprise in the attack. these two abilities only allow the thief to get into position unnoticed. The thief may stay unnoticed if he wishes or attack.

4. If he backstabs he must then role a seperate surprise role of 1-4 in 6. If he fails to win surprise its assumed the target suddenly senses the thieves attack and foils the backstab.

#4 is something that I believe I picked up from storm crow some time back, but not sure if its correct.

Additionally, a thief moving well in advance of his group surprises on a 1-4 in 6 regardless if he makes his MS role (as he's still considered to be moving more quietly and stealthy then untrained adventurers would be able to. The MS would allow the thief to sneak up unnoticed however, perhaps allowing him to back stab or return unnoticed to his main party to warn them of the targets ahead. Is this your take as well?

[Matthew]

1. The thief must attack from behind with a handheld weapon unnoticed.

Yes.

2. a MS role allows the thief to sneak up behind their prey, or a HIS allows the target to pass by the thief who can then back stab.

Not really. A successful move silently or hide in shadows roll allows a thief to be silent or virtaully invisible, which (as described in the thief function section) increases the chance of surprise.

3. A MS or HIS role made does not guarantee surprise in the attack. these two abilities only allow the thief to get into position unnoticed. The thief may stay unnoticed if he wishes or attack.

Yes.

4. If he backstabs he must then role a separate surprise role of 1-4 in 6. If he fails to win surprise its assumed the target suddenly senses the thieves attack and foils the backstab.

Very likely. Exactly what the surprise die should be (1-3, 1-4, 1-5, or even 1-6) should probably vary by circumstance, with 1-3 or 1-4 being most common.

Additionally, a thief moving well in advance of his group surprises on a 1-4 in 6 regardless if he makes his MS role (as he's still considered to be moving more quietly and stealthy then untrained adventurers would be able to. The MS would allow the thief to sneak up unnoticed however, perhaps allowing him to back stab or return unnoticed to his main party to warn them of the targets ahead. Is this your take as well?

No rule governs this, but it seems reasonable to allow a thief (or even lightly armoured adventurers) a higher chance of surprising the enemy if they take suitable precautions, which would be similar to the ranger and elf abilities.