Thieves and their ilk use the skills of Hide (Hide in Shadows) and/or Sneak (Move Quietly/Silently, Stealth) to help them remain undetected. Targeted characters can try to notice with the skills of Spot (Spot Hidden, Scan, Search) and/or Listen (Hear Noise) or even Smell (Sense). 

Hide and Sneak  The active party must roll a better degree of success (i.e. Critical/Special/Normal/Fail/Fumble) than the passive party. Draws give the party with the higher skill some minor advantage:
SPOT/LISTEN advantage :  suspicious they may have seen/heard something - may try to spot/listen again (actively) in subsequent rounds
HIDE/SNEAK advantage :  stop/duck back to avoid being seen/heard - may try again in subsequent rounds

Typical  Situations  The following rolls are required by participants in these frequently encountered situations:
HIDING :   motionless/only 5ft shift, in cover/shadow (for ambush/escape) - HIDE v SPOT (or Sense)
SNEAK UP :  from behind (to backstab/pickpocket) - SNEAK* v LISTEN (or Sense)
SNEAK AWAY :  from behind (to escape) - SNEAK* v LISTEN
SNEAK ASIDE :   from front/flank, not direct view (to escape line-of-sight) - HIDE/2** v SPOT  and  SNEAK* v LISTEN
SNEAK BY :  from front/flank, not direct view (to pass/get behind) - HIDE/2** v SPOT (or Sense)  and  SNEAK* v LISTEN (or Sense)
* SNEAK is usually at half move, else Difficult (/2).  ** HIDE is usually motionless, else Difficult (/2) up to half move, Auto-fail if faster.

Multiple Spotters/Listeners  Main roll gets bonuses from other rolls:  Success +10%, Special +15, Critical +20, Fumble -10. (Max x2).
Multiple Hiders/Sneakers  Main roll gets penalties from other rolls:  Failure -10%, Fumble -20%. (No limit to the penalty).

Surprise  Victims of successful sneaking attempts are surprised, which allows enemies one rounds worth of extra actions/attacks. 

Picking Pockets  When close to a potential target, whether unnoticed (e.g. by surprise) or unregarded (e.g. in a crowd), the Sleight skill (a.k.a. Pick Pockets, Sleight-of-Hand) gives a chance to steal something from them. A successful Sleight skill roll obtains an item (randomly selected by the GM, unless a particular item's location is known), but the target may notice, according to the Sleight roll's degree of success: Success INTx3; Special INTx2; Critical INTx1; Failure INTx5; Fumble Automatic (and embarrassing).

Backstabbing  When attacking by surprise from behind in melee, Sleight skill allows extra backstab attacks: each full 20% gives +1 attack. Weapons of up to Speed Factor 1 per 10% Sleight skill can be used. NB: Rear attacks are usually Easy (x2), cannot normally be parried, and the victim's dodging with Defence is Difficult unless they can make a Difficult Listen/Sense skill roll.

Idea/Perception Rolls  The GM may sometimes call for "INT rolls": Players should roll d100 and state the INT-multiple they achieved (x1, x5, or whatever).  It may be good enough to let their characters notice/remember relevant details - or to notice a pick-pocket!

Climbing  Characters making a successful Climb skill roll can climb at half their normal movement rate (i.e. the usual move-action distance, but per full round). Special makes climbing next round Easy; Critical Very Easy; Fail = No progress; Fumble = Fall.

Jumping Characters can jump horizontally as far as their Movement Rate (MOV) in feet, modified by a Jump skill roll: +/-1ft per 10% by which it is made/failed.  Distance is halved for vertical/standing-start jumps. Falling damage can be reduced by Jump/Acrobatics skill rolls: Success allows armour to apply; Special also reduces damage by d6; Critical also halves any remaining damage. NB: Jump skill is affected by armour burden/encumbrance/fatigue.

Fast Talk  Use of this skill requires just a minute or two of talking. It can be used against one or a few individuals. If successful, the victim will be confused into agreeing with whatever the fast talker says and do as they reasonably suggest - but the effect lasts only 5-10 minutes.

Oratory  Oratory (a.k.a. Persuade) may require hours of talking. It can be used upon one or many individuals. If successful, the listeners will be swayed emotionally into agreeing with particular points of the orator's speech and act accordingly - and the effect lasts indefinitely.

Bargaining  This skill is used when trading in order to get a discount on goods when buying, or a higher price for goods when selling. Success sells for 10% over standard price or buys for 10% under asking price (Special +/-25%, Critical +/-50%, Fumble -/+50%); Failure has no effect. But note that the other party may also Bargain, and merchants/traders could easily have about 50% Bargaining skill...

Bardic Voice  This skill is usually restricted to trained Bards. It is used in combination with Singing/Playing a musical instrument. One such skill must first be successfully performed, then effective use of Bardic Voice can affect the emotions of the listeners (up to 1 per 10% skill):
Calmness: Counters sonic effects such as siren's song, banshee wail, shrieker, etcetera.
Ferocity: Allied creatures within hearing nearby are inspired with increased morale (+1) and damage (+1).
Fascination: Even aggressive enemies will be enthralled by the song/tune (up to 1 minute per 10% skill). Save applies (WISx5). Special success allows the Bard to give a Suggestion, Critical success works like a Charm.

Bardic Lore  This skill can be used by a Bard to recall information, from their repertoire of ballads and songs, about particular famous and/or legendary subjects (including heroes and otherwise famous people, artifacts and major magic items, or noteworthy places and times).

BRP Ambushes: Stealth v Listen/Sense/Spot (or Search, if active). Missiles (unseen): Free round of attacks, Easy, no parry or dodge. Missiles (seen): Free round of attacks, Easy, can parry with shields or dodge. Melee (no stealth): target Easy Listen/Sense/Spot to notice. Fail - attacks Easy, parry/dodge Difficult. Melee (target aware, via Listen/Sense/Spot): target may parry/dodge, but not attack first round.