The second of three US presidential debate takes place on Sunday evening, two weeks after Hillary Clinton and Donald Trump sparred for the first time.

Plenty has happened in the meantime, including the explosive release of a video that has plunged the Republican nominee into a crisis.

And there have been plenty of hints what might come up in what is likely to be another bruising encounter.

Here is our A-Z guide to what is worth keeping an eye out for.

A is for Apology

Media playback is unsupported on your device Media caption Trump apologises for lewd comments and vows to be a "better man"

It's the only place to start, really. A tape from 2005, that emerged on Friday, showed Mr Trump making lewd comments about women, and saying he tried to kiss and grope them. He made a videotaped apology hours later, but the backlash even from within his own party was intense. Mrs Clinton has been gifted a line of attack, so how much of the debate can we expect to focus on what Mr Trump said, and how will he respond?

B is for Bingo

Good luck listening out for these terms...

C is for Crackling

Not the pork kind, but the audio kind. Mr Trump complained of a faulty microphone after the first debate - and it turned out his complaints were justified. Organisers will be keen to show that no candidate is disadvantaged this time around.

D is for Donald

It's what Mrs Clinton called her rival all the way through the first debate - he apparently does not like being called by his first name. So will she do the same again, or did that trick do its job?

E is for Environment

Climate change is a subject that has been mentioned only very very briefly in the two debates so far. Will it get any hearing in the town hall format?

F is for Foundation

On one hand: Mr Trump's charitable organisation, and the fact it was banned from fundraising in his native New York this week. On the other: Mrs Clinton's, and the concerns over its links to international donors. Trump, perhaps surprisingly, did not make a point of this in the last debate: will he now?

G is for Gennifer Flowers

Media playback is unsupported on your device Media caption Why Trump will not attack Hillary over Bill's affairs

...and the other women with whom Bill Clinton was unfaithful, or is alleged to have been. The New York Times reported last week that Mrs Clinton engineered an "aggressive strategy of counterattack" against the women in question.

It's an issue that is hurting Mrs Clinton among younger, female voters - so how would she respond to a question on this?

Read more: Hillary's campaign and Bill's women

H is for 'Hit her harder'

On a related note, Mr Trump has warned he may go for the jugular this time around. He didn't hit Mrs Clinton on some of the issues we might have expected - so will issues like Benghazi, her email scandal, the Clinton Foundation and her husband's infidelities get more of an airing?

I is for Interruptions

Even the most partial of observers would agree that there were more interruptions by Mr Trump last time around (the extent to which he interrupted changes from news outlet to outlet).

Similarly, Clinton deputy Tim Kaine was king of the interruptions on Tuesday night. Will Sunday's event be more civilised?

J is for Jokes

Don't hold out for any rolling in the aisles - neither candidate is known for their comic timing - but there's plenty of material there to play with.

K is for Knockout Blow

Media playback is unsupported on your device Media caption Key moments from the Clinton-Trump debate

More to the point, the lack of one - while there were plenty of zingers during the Hoftstra University debate, neither candidate was able to deliver that crucial fact or line that buried their opponent. Take two?

L is for Lies

Quite a few porkies were peddled last time around, so what should those in charge look out for? We asked a former FBI interrogator for his advice on weeding out the truth.

M is for Moderators

Yes, plural: there are two this time, CNN's Anderson Cooper and ABC's Martha Raddatz.

Trump was unhappy with the previous moderator, NBC's Lester Holt. There will be even more scrutiny this time around.

N is for No, That's Wrong

Just like last time around, we'll be keeping an eye on some of the statements made by both candidates. We're likely to be quite busy.

Read more: US Election Reality Check

O is for Ohio

And North Carolina. And Pennsylvania. And Florida. All the swing states that could decide this election, and the places where an impressive debate performance might help make a difference.

Media playback is unsupported on your device Media caption Election 2016: The race for battleground states

Media playback is unsupported on your device Media caption What are Clinton and Trump's best hopes of winning?

Read more: How does the election work?

P is for Polls

Who's leading in our poll of polls?

Please enable Javascript to view our poll of polls chart. About BBC poll of polls The BBC poll of polls looks at the five most recent national polls and takes the median value, ie, the value between the two figures that are higher and two figures that are lower. See individual polls

Q is for Questions

Unusually, ABC and CNN have said they are ready to consider 30 of the most popular questions submitted by the public on the website Presidential Open Questions, though there is no guarantee those questions will come up.

The most popular questions on the site at the moment relate to background checks for gun sales, benefits, the Second Amendment and term limits for members of Congress and the Supreme Court.

R is for Ratings

Clinton v Trump, Round One was watched by a record 84 million people - will we see the same audience this time around, or have people already had enough?

S is for Signs

Body language, specifically. We've taken a look at what both candidates' gestures say about them.

Media playback is unsupported on your device Media caption Decoding Trump's top five hand gestures

Media playback is unsupported on your device Media caption Breaking down Hillary Clinton's gestures

T is for Town Hall

It's a different format to last time around: half the questions will be asked by members of the public, and half by the moderators, based, according to the independent organisers "on topics of broad public interest as reflected in social media and other sources".

U is for Upper Hand

Clinton saw a jump in the polls after debate number one; the Trump campaign was buoyed by the performance of Mr Trump's right-hand man Mike Pence in his debate. Could this debate present one candidate with momentum, with only one more televised event to come?

V is for Veep

Last Tuesday saw the only vice-presidential debate between Mr Kaine and Mr Pence: the former went for the attack; the latter remained unflappable - almost the reverse of the senior candidates' performances in the first debate. Will Mr Trump be buoyed by Mr Pence's strong show?

Media playback is unsupported on your device Media caption US vice-presidential debate: Two styles of attack

Read more: Who won VP debate?

W is for Washington University

Is the venue of the debate, in St Louis, Missouri. It all starts at 21:00 eastern time on Sunday (01:00 GMT).

X marks that spot where you sign your tax return

(Tenuous, we know)

The New York Times reported this week that Mr Trump may not have paid federal income tax for 18 years, after posting a near-$1bn loss in 1995. The candidate said this showed he had "brilliantly" navigated the complex tax code.

If this subject doesn't come up, it will be a huge surprise.

Read more: How could Trump have avoided tax?

Y is for Youth Vote

There are now more millennials (aged 18-34) than baby boomers (aged 51-69) in the US, according to the Pew Research Center. That's a lot of potential voters. Mrs Clinton's one-time Democrat co-runner Bernie Sanders is popular with this age group, so she has a fight on her hands to lure them to her side and avoid seeing them vote for third-party or independent candidates.

The problem? The Trump campaign has picked up on an audio recording they say shows Mrs Clinton denigrating Sanders supporters. Democrats say it doesn't; listen to it and make your own minds up.

Z is for Zurcher

Our North America reporter, Anthony Zurcher, will be in Missouri - you can read all his analysis here and follow him on Twitter here.

Read more: Why this Trump row is different