Help: Brokers and Liquidity

Beginner Questions
1

Hi everyone, my question is:

– Are brokers / market-makers only fulfilling orders when there is another trader on the other end? E.g. They’re just the middlemen?
– Or, are they required to fulfil an order even in the absence of public trading, because they are “market makers”? E.g. They become the trader if there aren’t any traders?

If it’s the second, how could there ever be slippage?

My question applies to CFDs & Spread Bets on Forex, Indices, Shares – please advise if the answers are different depending on what you trade.

Thanks

2

I am spreadbetting, now on forex and indices but previously mosty on UK share prices. The SB companies are market makers, i.e. bookmakers, there is not a specific counterparty to an SB trade.