Many called it the 'War without enemies'. Cromwell told his men, on the eve of the invasion that the Scots were 'Our brethren who had been misled'. He also warned them, as he had in Ireland that to take anything from or do harm to any person not in arms would result in summary justice.
This often resulted in hanging but other punishment could be to 'ride the cannon (strapped to a cannon which was then fired) or to march twenty paces.
That was to be hanged and the normal reflex was that legs should 'march' twenty times and the prisoner cut down.
There are letters from opposing soldiers to one another mentioning the love and friendship that they had shared and no matter what happened in the Battle tomorrow, the love and regard would not change.
Cromwell sent a letter to a friend who had a daughter in a (closet) Roman Catholic girl's school, warning him that the school was to be attacked. This was prior to the New Modelling of the army, so there were no particular rules on soldier's behaviour.
It was a 'civil' war because Cromwell and Ireton tried honestly to bring Charles to a firm agreement, but Charles was the 'Arthur Daley of kings. He'd buy a dodgy car with a bent checque!
After beating the King at Naseby there were a series of meetings called 'The Putney Debates'. Soldiers elected by the ranks, Officers, Politicians all trying to determine what kind of country they wanted to live in. Cromwell has always been accused of Machiavellian cunning but the Putney Debates show Henry ireton as the political thinker and Cromwell as the Country Squire who was damn good with Cavalry and really just wanted a return to order with no illegal taxes (see ship money) Freedom of conscience in religion and Property remaining in the hands of the owners which really Peed off the Levellers. He also believed in the arch of state thich consisted of The Commons, the Lords and the King placed on top. Without one or the other, the structure would fall. Cromwell disappointed the Levellers, the Republicans, The Presbyterians and the Judges. Levellers because he knew it couldn't work and that the redistribution of wealth was always at the cost of someone who owned money and property. The Republicans because they wanted to dismantle the Arch of State and make the Commons the sole rulers of Britain in both Politics and Justice. They also wanted to give themselves the power to stay in place and new member would be selected by them. Sir Edward Hyde, said that if Cromwell hadn't driven those people out of Parliament, it would have taken ten times the blood to remove them than it had to remove the King. The Presbyterians were disappointed because they wanted to emulate the Taliban like clerics who had ruled Scotland so cruelly after the Covenant. They turned it from a wonderful thing to a (as Cromwell described it) A pact with the Devil and hell.
The Judiciary were unhappy because Cromwell came up with novel ideas like judges being paid a set wage and not able to set 'charges' at will. He wanted to clear the Debtors Prisons of people who could prove that they were unable to pay debts. The cost of living in a Debtors Prison often ran far above the value of the debt. Cromwell also disliked the fact that in the Court of Chancery (Chancers!) there were cases which had been dragging on for over thirty year. Other cases ran up costs that exceeded the original claim.
There is a book, and it is available by a man by the name of 'Innerwick' the book is called 'The Interregnum'. In order to re-enact the Civil war, it may be useful to re-live the arguments and the enlightenment of that foundling Parliament.
Best reads are.... C.V.Wedgewood...'Strafford'. Viscount John Morley for an unbiased biography on Oliver Cromwell. Thomas Carlile.... Letters and Speeches of Oliver Cromwell (4 volumes but I dip into them as much as I do Terry Pratchett books). John Buchan did an Oliver Cromwell and a 'Montrose'. The Cromwell book isn't all that good, but the 'Montrose' is excellent.