"Sacrificial lamb" and "scapegoat" don't seem to be completely interchangeable. What are the difference?
Some articles online, for instance this one here, that touch on this topic talk about how these two terms are similar. The aforementioned article defines scapegoat as
A scapegoat is someone who dies for the benefit of a society, who takes on society’s collected sins.
Wiki pages say:
A sacrificial lamb is a metaphorical reference to a person or animal sacrificed for the common good.
and
the word "scapegoat" has developed to indicate a person who is blamed and punished for the sins of others.
Of course, scapegoat by origin has a heavier tone of sinfulness associated with it. But the modern figurative meanings seem to overlap significantly, if not completely.
Best Answer
The ‘scapegoat’ first appears in the Old Testament book of Leviticus. This refers to an annual ritual in which the community loaded their sins onto a goat and drove it into the desert (to its death). There have been rituals like this all over the world. In ancient Greece we hear of all sorts of nasty things being done to some poor criminal each year.
So, properly speaking, a scapegoat is a person used to assume guilt that belongs to others.
The original sacrificial lambs must have been those, in the Book of Exodus, that had their blood daubed on the lintels of the children of Israel on the eve of their flight from Egypt. They were sacrificed so that the angel of the Lord would know from this sign not to kill the firstborn in those households. So a sacrificial lamb is essentially innocent.
So the two differ slightly. A scapegoat is more likely to be someone who shares in the guilt for something, but but us used to take the whole blame by other more powerful.
The sacrificial lamb can be sacrificed in all sorts of contexts. Wikipedia mentions how a new politician might be entered for an unwinnable seat in a general election. So it could be unconnected with guilt or innocence altogether.
But the two are close enough for there to be scope for ‘leaching’ between the two. So ‘sacrificial lamb’ could be used where ‘scapegoat’ would be appropriate, but not, I think, vice versa.
However