Can you put a fielder straight?
While the ball is in play and until the ball has made contact with the striker's bat or person, or has passed the striker's bat, no fielder, other than the bowler, may have any part of his/her person grounded on or extended over the pitch.
A fielder can not sand staraight behind the wicket keeper. If any fielder stands so then it will be called as a no ball.
Standing in front gives the keeper more room without having to run around the stumps. Also, if the throw is weak and accurate, sweeping your hands through the line of the stumps (with your foot as a giude) is faster than waiting for the ball. Most successful direct hits do not have the keeper involved.
The captain (usually in consultation with the bowler and sometimes other members of the team) may move players between fielding positions at any time except when a bowler is in the act of bowling to a batter, though there are exceptions for fielders moving in anticipation of the ball being hit to a particular area.
Walking in keeps momentum going forward. Move continually towards the ball prevents a quick single or allows you to cut the firmly struck ball off before it is past you.
19.5. 2 A fielder who is not in contact with the ground is considered to be grounded beyond the boundary if his/her final contact with the ground, before his/her first contact with the ball after it has been delivered by the bowler, was not entirely within the boundary. 19.6.
Fake fielding in Cricket is defined as the “deliberate distraction, deception, or obstruction of batsman.” According to Cricket Law 41.5, cricketers are prohibited from misleading, deceiving, or impeding any batter after the striker has received the ball by using words or actions.
Other than the pitcher and catcher, all other fielders may position themselves anywhere in fair territory.
In all forms of cricket, only two fielders are allowed in the quadrant between the fielding positions of square leg and long stop. This is to prevent the outlawed and controversial bodyline tactics from being used. No fielder is allowed on or over the pitch until the batsman has had a chance to play the ball.
19.5. 2 A fielder who is not in contact with the ground is considered to be grounded beyond the boundary if his/her final contact with the ground, before his/her first contact with the ball after it has been delivered by the bowler, was not entirely within the boundary. 19.6.
What happens if a runner runs into a fielder?
A runner may slide into the fielder. a. When a runner is called out for crashing into a fielder holding the ball, the ball becomes dead. Each runner must return to the last base touched at the time of interference.
One of cricket's golden rules is that fielders stay silent from the moment a bowler starts their run-up until a batsman completes his shot.

References
- https://baseballrulesacademy.com/official-rule/ncaa/ncaa-5-3-positions-of-the-defensive-team/
- https://www.lords.org/mcc/the-laws-of-cricket/boundaries
- https://en.wikipedia.org/wiki/Fielding_(cricket)
- https://cdn4.sportngin.com/attachments/document/0127/7121/53f347_ea70c33641394343b5d86398e17dc8bd.pdf
- https://www.pitchvision.com/wicketkeeping-in-front-of-the-stumps
- https://www.quora.com/Can-a-fielder-stand-behind-straight-to-the-wicket-keeper
- https://en.wikipedia.org/wiki/Fielding_restrictions_(cricket)
- https://www.pitchvision.com/avoid-being-left-behind-when-walking-in
- https://www.jagranjosh.com/general-knowledge/what-is-fake-fielding-in-cricket-the-rules-explained-1667458812-1
- https://www.smh.com.au/sport/cricket/indian-fielders-talked-during-runups-20170321-gv2sl0.html
- https://www.lords.org/mcc/the-laws-of-cricket/the-fielder