It depends on if you are adding or removing liquidity from the market. i.e., if you are placing limit or market orders.
Order books are divided into asks and bids. An ask is the lower price that someone is willing to sell their asset for, and a bid is the highest price that someone is willing to pay for an asset.
An order is executed when an ask is matched against a bid, or vice-versa. If you enter an order for which the corresponding liquidity is already available, you are effectively placing a Market order - It will be executed immediately.
If you enter an order that is far enough from the ask/bid to execute immediately, it is a limit order. You will end up with an entry in the order book, and someone has to place an equivalent order at your price (but in the opposite direction) before your trade goes through.
For example, Alice has 1 BTC and is willing to sell it for $1000. Bob has $990, and is willing to buy 1 BTC.
The order book in this case has an ASK of $100, and a BID of $990. Since there is no match between the ask and the bid, no trade happens. These are limit orders, which means they will only execute at that price.
Now John comes in and puts an order to buy 0.5 BTC at $1000. Since Alice is already offering 1 BTC at this price, half of her order is filled, and all of John's order is filled immediately. Since this is removing liquidity, it is essentially a market order (even though it was placed as limit). The order book is now ASK 0.5 BTC @1000, and BID 1 BTC @ 990.
Now let's say Smith comes in and places a market order for 1 BTC. When placing a direct market order, you don't specify a price. You will automaticaly trade at the lowest ASK/highest BID depending on the direction of your trade.
Since our order book only has 0.5 BTC available for sale, the 1 BTC market order only fills 50%.
In short, the price depends on the type of order you use, and how much liquidity there is in the market. The spot price is simply the last price a trade was executed at, and there is no guarantee that there is liquidity available at the spot price (the last trade may have consumed all the liquidity).