There are three circumstances in which the Prime Minister forms a government.
The Prime Minister will not know whether people who have been impressive in Opposition will be successful as Ministers.
As shadow ministers they will have led on a policy area in Parliament but the Prime Minister may shift them to new roles.
For example, Robin Cook was the Shadow Health Secretary in Opposition but, on coming to office in 1997, Tony Blair moved him to the post of Foreign Secretary because they did not agree on public service reform.
A Prime Minister retires
A Prime Minister may retire and the new Prime Minister will take over without an election.
The new Prime Minister will have different views on priorities and on senior colleagues.
For example, when James Callaghan took over from Harold Wilson in 1976 he removed Barbara Castle. Why? Even though she had been one of the key figures in the Government, they had never seen eye to eye on many issues.
Prime Ministers regularly change their Government while in office. This is to rejuvenate them, move less successful ministers out and bring new people on. There may also be an unexpected resignation that means a Minister has to be replaced.
What factors will the Prime Minister take into account in forming a Government
Key figures, factions and rivals
Key figures in the Parliamentary Party will need to be included.
Some of these will be close allies of the Prime Minister, as George Osborne and Michael Gove were with David Cameron.
Others may be major figures with support in the Party such as Theresa May and Iain Duncan-Smith in David Cameron’s Government and Robin Cook and Donald Dewar, who saw through Scottish devolution, were in the Labour Government of 1997.
The Prime Minister may actually want potential rivals for his position to be in the Government. Why? Because under Cabinet Collective Responsibility they cannot then criticise policies in public.
This isn’t always easy though. For example, Tony Blair never sacked Gordon Brown despite the conflicts between them. When Michael Heseltine resigned from Margaret Thatcher’s Government he became a focus for opposition on the backbenches and eventually stood against her for the leadership.
The Prime Minister needs to balance the various factions in the party to make sure that they are represented in the Government.
For Labour this has meant including both right and left wings of the party, although under Blair, with Gordon Brown very powerful, this instead meant including Blairites and Brownites with personal loyalty being more important than policy differences.
Margaret Thatcher had to include those who were not keen on her right-wing economic policies when she took office and only when she felt much more secure was she able to dispense with them.
Cameron included some more Eurosceptic ministers to satisfy the right of the party in the run up to the EU Referendum. Theresa May had to balance Brexiteers and Remainers following the 2016 EU Referendum.
The Prime Minister also needs to ensure that MPs from different parts of the country are represented.
Now, that there are more women and MPs from minority groups, there is some gender and ethnic balance in the government. The aim is to appear more representative of the country.
Are they good at their job and loyal?
Over time a government reshuffle becomes necessary to remove ministers who do not seem to have been competent or who may have shown some disloyalty to the Prime Minister.
Blair removed Robin Cook from the Foreign Office in 2001 because he was unlikely to support his policy on Iraq but still kept him in the Government so that he was not a critic on the backbenches .
A reshuffle makes it possible to promote new people who are talented, including those recently elected to Parliament or to reward loyalty.
The Chief Whip will play an important role in this as the whips, unlike the Prime Minister, are in Parliament all the time talking to MPs and observing how well they perform in debate.
Even though a reshuffle allows the PM to exert power, holders of the office such as, Blair and Cameron, actually dislike the process of sacking Ministers and have kept their reshuffles to a minimum. Theresa May had more reshuffles due to Brexit resignations rather than choice.
Prime Ministers often need to give the government a new look, especially as an election nears.
Cameron’s pre- 2010 election reshuffle removed Ken Clarke who had been in Conservative Governments for some thirty years. He promoted new faces, including more women, who would be appearing on television to promote Government policies in the run-up to the election.