It's pretty common to see people make a tiny bet on the flop or the turn, and if you just call it, they think you're weak and make a big bet on the river to take the pot away. Just calling these weak bets can be a good way to induce a river bluff. If I'm Player 2 in Hand 1, I call with a pretty decent range of hands (although I probably would have raised the flop with most of those hands).

Example: I'd seen this guy float 3 or 4 times already with ace high, and my HUD said he'd "folded to flop bet" something like 4 out of 15 times. When he made his little $10 bet on the turn, I gave him zero credit for a hand, so I called planning to call another bet on the river if it was a blank. Thanks for the stack!

POKERSTARS GAME #4855231712: HOLD'EM NO LIMIT ($1/$2) - 2006/05/06 - 22:25:45 (ET)
Table 'Astyanax III' 6-max Seat #4 is the button
Seat 1: snapper10k ($126.80 in chips)
Seat 2: pinocchio2 ($428.80 in chips)
Seat 3: mcatdog ($200 in chips)
Seat 4: Smut Mut ($118.90 in chips)
Seat 5: Pryderi ($196 in chips)
Seat 6: nicenick ($80 in chips)
Pryderi: posts small blind $1
nicenick: posts big blind $2
*** HOLE CARDS ***
Dealt to mcatdog [Jd Jh]
snapper10k: folds
pinocchio2: folds
mcatdog: raises $6 to $8
Smut Mut: calls $8
Pryderi: calls $7
nicenick: calls $6
*** FLOP *** [Kd 3s 4h]
Pryderi: checks
nicenick: checks
mcatdog: bets $20
Smut Mut: calls $20
Pryderi: folds
nicenick: folds
*** TURN *** [Kd 3s 4h] [2s]
mcatdog: checks
Smut Mut: bets $10
mcatdog: calls $10
*** RIVER *** [Kd 3s 4h 2s] [7h]
mcatdog: checks
Smut Mut: bets $80.90 and is all-in
mcatdog: calls $80.90
*** SHOW DOWN ***
Smut Mut: shows [2d Ah] (a pair of Deuces)
mcatdog: shows [Jd Jh] (a pair of Jacks)
mcatdog collected $250.80 from pot
*** SUMMARY ***
Total pot $253.80 | Rake $3
Board [Kd 3s 4h 2s 7h]
Seat 1: snapper10k folded before Flop (didn't bet)
Seat 2: pinocchio2 folded before Flop (didn't bet)
Seat 3: mcatdog showed [Jd Jh] and won ($250.80) with a pair of Jacks
Seat 4: Smut Mut (button) showed [2d Ah] and lost with a pair of Deuces
Seat 5: Pryderi (small blind) folded on the Flop
Seat 6: nicenick (big blind) folded on the Flop